The present invention relates to an improved magnetic recording material, and more particularly relates to improvement in mechanical property of a magnetic recording material having anodic oxide layers and well suited for perpendicular magnetic recording.
A magnetic recording material of such a type is disclosed in, for example, Japanese Patent Publication Sho.51-21562, in which anodic oxidation is applied to a substrate made of Al or Al base alloy and magnetic substance is packed in fine pores in an anodic oxide layer on the substrate. Further, other examples are found in, for example, U.S. Pat. Nos. 4,650,708 and 4,681,669 as well as 1MB Technical Disclosure Bulletin Vol. II, Nos. 3, AUG., 1968.
In the case of such a magnetic recording material, the thinner the anodic oxide layer packed with the magnetic substance, the better the magnetic recording density (D.sub.50) and the overwrite characteristics (O/W). However, a too thin construction of the anodic oxide layer reduces the output voltage (Vout) from a magnetic head and the strength of a magnetic layer. The optimum thickness of the anodic oxide layer is said to be in a range from 0.5 to 1.5 .mu.m.
With the conventional magnetic recording material of this type, however, reduction in thickness of the anodic oxide layer lowers the mechanical strength and surface hardness, thereby degrading durability of the magnetic recording material. This is believed to be caused by the fact that the apparent rigidity of the relatively hard anodic oxide layer is impaired under influence of the substrate made of relatively soft Al or Al -base alloy. At starting and ending of rotation of a magnetic recording material, a magnetic head comes in abrasive contact with its anodic oxide layer. In addition, slight warp and/or vibration of the magnetic recording material causes such abrasive contact also. In order to withstand such abrasive attack, a magnetic recording material is required to have sufficient mechanical strength and surface hardness.
Despite such an intense demand, it is very difficult to reduce the thickness of the anodic oxide layer for the above-described reason. Use of relatively soft Al or Al -base alloy for the substrate additinally necessitates thick construction of the anodic oxide layer in order to cover poor flatness of the substrate. For these reasons, it is conventionally difficult to reduce the thickness of the anodic oxide layer below a range from 1.5 to 2 .mu.m.